A gradient photochromic glass article is a glass article exhibiting a gradient in photochromic properties thereacross. One example is a photochromic glass article having a first region exhibiting photochromic properties normal for a photochromic glass and a second region exhibiting photochromic properties which are at least substantially less developed than those of the first lens portion. Often the second portion is non-photochromic. The transition from the strongly photochromic to the weakly photochromic or non-photochromic region may be gradual or abrupt.
A number of methods for producing gradient photochromic glasses are known. Canadian Pat. No. 739,404, for example, describes a method wherein potentially photochromic glass is heat treated in a partitioned lehr having a high temperature zone and a low temperature zone. Alternately, the glass is partly encased in a refractory block during heating. Such heat treatments establish a temperature gradient across the glass which is effective to provide a gradient in photochromic properties thereacross. Thus potentially photochromic glass in the high temperature zone of the lehr reaches a temperature sufficient to develop photochromism therein while the glass in the low temperature zone does not.
A related method for producing gradient photochromic glass is described in German Patentschrift 2,125,232. According to that method, a continuous sheet of potentially photochromic glass is drawn past heating means which selectively heat only part of the width of the glass sheet. The heated width develops photochromic properties whereas the remainder of the sheet width does not.
As presently proposed, neither of the foregoing two methods is suitable for the manufacture in quantity of discrete lens blanks for gradient photochromic ophthalmic lenses. The former method involves a special lehr and/or individual handling of each piece to be treated, whereas the latter method is useful only for the treatment of continuous sheet. Other processes for producing gradients in photochromic properties across glass articles, such as the ion-exchange method described in U.S. Pat. No. 3,419,370, are similarly unsuited for the manufacture of gradient photochromic ophthalmic lenses.
One method recently developed for the production of gradient photochromic ophthalmic lens blanks comprises subjecting potentially photochromic glass blanks to a heat treatment while maintaining part of the lens blank in proximity to a quantity of evaporable liquid contained in or supported by a porous refractory carrier material. The heat treatment is carried out at a temperature sufficient to develop photochromic properties in the potentially photochromic glass; however, vapor generated by the evaporable liquid during heat treatment maintains the part of the lens blank in proximity thereto at temperatures below that required for full photochromic development. Thus a gradient photochromic ophthalmic lens blank is provided.
Certain disadvantages arise in the production of gradient photochromic ophthalmic lens blanks by the above-described method. One major disadvantage is that porous refractory carrier materials of the kind best suited for controlled vapor release are subject to deterioration and breakage due to the adverse physical and chemical effects of repeated thermal cycling in the presence of liquid.
A very important requirement of any glass product intended for ophthalmic use is uniformity with respect to clarity, color, and photochromic response. Gradient photochromic ophthalmic lens blanks must meet a further requirement of uniformity in that the quality (relative sharpness) and position within the lens blank of the photochromic gradient must be reproducible. In the vapor process, it appears that rather minor variations in refractory carrier porosity and/or liquid absorption may produce variability in the gradient photochromic product.
It is the principal object of the present invention to provide a process for producing a gradient photochromic ophthalmic lens blank which is not subject to the above-described disadvantages.
It is a further object to provide a process which produces gradient ophthalmic lens blanks of improved uniformity with respect to gradient quality.
Other objects and advantages of the invention will become apparent from the following description and examples thereof.